1. Field of the Invention
This invention relates to a system for detecting a prospective abnormal shadow in a radiation image on the basis of image data representing the radiation image. This invention also relates to a method of and an apparatus for judging whether a prospective abnormal shadow is malignant or benignant.
2. Description of the Related Art
In the medical field and the like, there are cases where radiation image data is obtained from a radiation image of an object recorded on a photographic film, stimulable phosphor sheet and the like, suitable image processing is carried out on the image data and an image is reproduced on a display on the basis of the processed image data. Recently there have been developed various computer-aided digital image processing techniques. Also, various image input devices (image forming modality) such as a CT (computed tomography) apparatus, a MRI (magnetic resonance imaging) apparatus, a CR (computed radiography) apparatus and the like are used for forming diagnostic images.
The digital image processing techniques are basically different from the conventional analog image processing techniques in that the digital image processing techniques can quantitatively analyze image data. For example, there has been proposed an abnormal shadow detection processing technique called CADM (computer-aided diagnosis of medial image) which detects an abnormal shadow representing a growth, a micro calcification or the like by positively making use of the merit of the digital image processing technique. See, for instance, “Growth Shadow Detection in a DR image (Iris Filter)” (Journal of Academy of Electronics/Information/Communication D-II, vol. J75-D-II No. 3, pp663 to 670, March 1992) and “Extraction of Micro Calcification Shadow by a Morphology Filter using multiple Structural Elements” (Journal of Academy of Electronics/Information/Communication D-II, vol. J75-D-II No. 7, pp1170 to 1176, July 1992).
In the abnormal shadow detection processing, the digital image signal is analyzed by a computer to automatically detect a prospective abnormal shadow which is conceivable to represent a growth, a micro calcification or the like. The detected prospective abnormal shadow is marked to call the attention of the reader. Alternatively, the probability that the prospective abnormal shadow represents a genuine abnormality or the feature value or the index on the basis of which a part of an image is determined to be a prospective abnormal shadow is output, whereby a certain detecting level can be ensured irrespective of the skillfulness of the reader, and overlooking or misreading can be suppressed.
We, this applicant, have proposed a prospective abnormal shadow detecting system, for instance, in U.S. Pat. No. 5,761,334 and Japanese Patent Application 2000-038298, in which a prospective abnormal shadow indicating existence of, for instance, mammary cancer is automatically detected by the use of an iris filter or a morphology filter on the basis of a mammogram or a breast radiation image and the prospective abnormal shadow part of the image is enhanced or enlarged. Then the processed (e.g., enhanced or enlarged) prospective abnormal shadow part is displayed over the overall image or the probability that the prospective abnormal shadow represents a genuine abnormality and/or the feature value or the index on the basis of which the part of the image is determined to be a prospective abnormal shadow is displayed in addition to the overall image.
When the iris filter is used, the density gradients (or the brightness gradients) in the image are represented by gradient vectors and a part of the image where the degree of convergence of density (signal value) gradient vectors is high is determined to be a prospective abnormal shadow, whereby a prospective shadow of growth inherent to the breast cancer can be automatically detected. In the processing using the morphology filter, apart which fluctuates in density in a space narrower than multiple structural elements selected according to the size of the abnormal shadow to be detected is determined to be a prospective abnormal shadow, whereby a prospective shadow of micro calcification inherent to breast cancer can be automatically detected.
We, this applicant, have further proposed improving accuracy of the prospective abnormal shadow detection processing by detecting the prospective abnormal shadow using an evaluation function value such as a Mahalanobis distance as disclosed, for instance, in U.S. Pat. No. 5,732,121.
However, though outputting a prospective abnormal shadow portion such as a prospective growth shadow and a prospective micro calcification shadow, the previously proposed abnormal shadow detection techniques do not output information on whether the abnormality represented by the prospective abnormal shadow is malignant or benignant.
For example, if only the prospective abnormal shadow detection processing using the iris filter or the morphology filter or additional processing for improving the accuracy is carried out, the prospective abnormal shadows representing the malignant and benignant abnormalities are both enhanced.
This can deteriorate the diagnostic efficiency since prospective abnormal shadows representing unmalignant abnormalities are output together with those representing malignant abnormalities.
When prospective abnormal shadows are detected by the use of the iris filter or the morphology filter, prospective abnormal shadows which are conceivable to be malignant are separated from those which are conceivable to be benignant (will be respectively referred to as “malignant prospective abnormal shadow” and “benignant prospective abnormal shadow”, hereinbelow) on the basis of a morphological feature, and only malignant prospective abnormal shadows are detected as final prospective abnormal shadows as disclosed in U.S. Pat. No. 5,761,334, Japanese Patent Application No. 2000-038298, and “Distinguishment between benignant and malignant micro calcification clusters in a breast X-ray image”; papers of medical image engineering academy “JAMIT Frontier '99 pp. 89 to 93, only malignant prospective abnormal shadow is output as an image and benignant prospective abnormal shadow is not output as an image. However, since a benignant abnormality can change to a malignant abnormality, it is not preferred that only malignant prospective abnormal shadow be output as an image but it is rather preferred that a suggestion to the effect that the part corresponding to the benignant prospective abnormal shadow is to be kept watched be output together with the image of the malignant prospective abnormal shadow.
That is, none of the conventional systems outputs information sufficiently effective to diagnosis as a result of the automatic detection of the prospective abnormal shadow. That is, in order to enhance effectiveness of the result of the automatic detection of the prospective abnormal shadow as information for assisting the reader in reading the radiation image, it is preferred that information on whether the prospective abnormal shadow is malignant or benignant be output.